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Cysteine cathepsins are peptidases with housekeeping functions that play different specific roles in different tissues. Endogenous peptidase inhibitors, such as cystatins and thyropins are the ultimate way of controlling their activity. It appears, however, that cathepsin X, a monocarboxypeptidase, whose overexpression is associated with several pathological processes, is not under the control of endogenous inhibitors. Inhibitors belonging to various groups inhibit other cathepsins tested, but none decrease the carboxypeptidase activity of cathepsin X. This absence of inhibitor control is another feature that distinguishes cathepsin X from other members of the cysteine peptidases.